1. Field of the Invention
This invention relates to an apparatus for projecting and exposing a semiconductor circuit pattern, a liquid crystal display element pattern or the like onto a photosensitive substrate.
2. Related Background Art
In a projection exposure apparatus of this kind, the exposure of a reticle pattern has heretofore been effected with the surface of a photosensitive substrate (a semiconductor wafer or a glass plate having a resist layer applied thereto) disposed on the best imaging plane of a projection optical system (a plane conjugate with the reticle).
However, the area exposed once on the wafer (the shot area) is of the order of 15 mm square to 20 mm square, and if the surface of the wafer is minutely curved in that area or there is unevenness of the order of several .mu.m in the surface structure, there will appear in the shot area a portion which exceeds the depth of focus of the projection optical system. This is because the depth of focus of the projection optical system is only .+-.1 .mu.m or so on the image side (the wafer side).
So, a method whereby exposure can be effected at an effectively wide depth of focus even in an exposure apparatus having a projection optical system of a small depth of focus has been proposed in U.S. Pat. No. 4,869,999. In the method disclosed in this patent, a wafer is moved to two or three points in the direction of the optic axis of the projection optical system and one and the same reticle pattern is multiplexly exposed at each point. In this method, two points distant in the direction of the optic axis are made nearly as wide as the depth of focus .+-..DELTA.Z of the projection optical system, thereby enlarging the actually effective depth to the order of 1.5-3 times.
In addition to the method as described in the above-mentioned patent wherein the wafer is positioned at each of multiple points in the direction of the optic axis and exposure is repeated, there has been proposed a method whereby the wafer is continuously moved (or vibrated) in the direction of the optic axis during an exposure operation for one shot area.
A semiconductive integrated circuit is manufactured by the steps of film formation, pattern transfer, etching, etc. being repeated several to ten and Several times. Therefore, in some cases, portions in which film corresponding to several layers is laminated and portions in which no film is laminated are mixedly present on the surface of a wafer which is in the process of forming an integrated circuit. The thickness of a layer in the film is of the order of 0.1 .mu.m to 1 .mu.m, and the level difference on the wafer surface (in one shot area) may be of the order of several .mu.m at greatest. On the other hand, the depth of focus of the projection optical system is generally expressed as .+-..lambda./2.multidot.NA.sup.2, where .lambda. is the wavelength of illuminating light for exposure, and NA is the numeral aperture of the image plane side of the projection optical system. In the latest projection optical systems, .lambda.=0.365 .mu.m (the i-line of a mercury lamp) and NA.congruent.0.5, and the depth of focus .DELTA.Z in this case is about .+-.0.73 .mu.m.
Accordingly, when as in the prior art, exposure is effected with a wafer fixedly disposed on the best imaging plane of the projection optical system, both the top and bottom of the level difference on the wafer become distant in the direction of the optic axis by more than the depth of focus of .+-.0.73 .mu.m relative to the best imaging plane (the best focus plane) and thus, image formation becomes impossible.
Also, according to a method whereby as in the prior art, exposure is effected plural times with a wafer positioned at multiple points separate in the direction of the optic axis, it is possible to cope with a level difference of several .mu.m on the wafer, but a shutter system must be driven to stop and resume exposure at multiple points in the direction of the optic axis, and this has led to the problem that the wafer treating ability per unit time is greatly reduced under the influences of the driving of a drive stage (Z stage) in the direction of the optic axis (Z direction) of a wafer holder, the positioning operation characteristic and the opening and closing of the shutter.
So, an example of the prior-art exposure method will hereinafter be described with reference to FIGS. 1A-1C of the accompanying drawings. FIGS. 1A and 1B show the time charts of the shutter operation (the variation in the illumination on a wafer) and the Z stage operation when multiplex exposure is effected at two focus positions to one shot area, and FIG. 1C shows the time chart of the shutter operation in a normal mode in which multiplex exposure is not effected.
Here, it is to be understood that during multiplex exposure and during normal exposure, the same exposure amount is provided to one shot area on a wafer. In the case of normal exposure, assuming that the operation time Ta until the closed shutter is opened and the operation time Tb until the shutter is closed from its fully open state are of a substantially equal value (Tc), a proper exposure amount BV is BV=(Tc+T.sub.O ').times.IL, where T.sub.O ' is the fully open time of the shutter, and IL represents the illumination of the surface of the wafer. Also, in FIGS. 1A and 1C, the ordinate OP. represents the fully open state of the shutter and CL. represents the fully closed state of the shutter. Further, in FIG. 1, time Tst represents the stepping time to the next shot area of the wafer stage.
On the other hand, in the case of multiplex exposure, the first exposure is effected with the Z stage set at a position +Z.sub.1 upwardly distant from the best focus plane. Z.sub.0 as shown in FIG. 1B, whereafter the Z stage is re-set at a position -Z.sub.1 downwardly distant from the plane Z.sub.0 during a time T.sub.Z, and then the second exposure is effected.
The operational characteristics (rising and falling) of the shutter do not vary as long as one and the same exposure apparatus is used and therefore, the first exposure time is Ta+Tb+T.sub.0, where T.sub.O is the fully open time of the shutter, and if the first exposure amount is about one half of the proper exposure amount BV, the fully open time T.sub.O is defined as follows (but BV/IL=Tc+T.sub.O '): ##EQU1##
Thus, as is apparent from Figures assuming that Ta=Tb=Tc, in the case of normal sensitization, the overall treatment time which gives the proper exposure amount BV to each shot area on the wafer is EQU (2Tc+T.sub.O '+Tst).times.the number of shots (1)
and in the case of multiplex (two times) exposure, said over all treatment time is EQU {2(2Tc+T.sub.O +T.sub.Z +Tst}.times.the number of shots (2)
Substituting T.sub.O =1/2(T.sup.O '-Tc) for this expression (2) and rearranging it, EQU (3Tc+T.sub.O '+T.sub.Z +Tst).times.the number of shots. (3)
So, comparing expressions (1) and (3) with each other, it will be seen that in the case of multiplex exposure, the time becomes longer by (Tc+T.sub.O) during each shot.
In the present-day exposure apparatuses (steppers), the time Tc is 10-30 mSec. and the time T.sub.Z, although it differs depending on the strokes of positions +Z.sub.1 and -Z.sub.1, is of the order of 20-50 mSec. Therefore, the time becomes longer by the order of 30-80 mSec. during each shot, and assuming that there are 100 shot areas on a wafer, the treatment of one wafer may become longer by 3 to 8 sec.
As described above, the prior-art method has suffered from a problem is the throughput of wafer treatment.